Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A separator for a redox flow battery and a redox flow battery including
the same, and the separator includes a cation conductive film and an
anion conductive film disposed on either side of the cation conductive
film.

Claims:

1. A separator for a redox flow battery, comprising: a cation conductive
film; and an anion conductive film on either side of the cation
conductive film.

2. The separator of claim 1, wherein the anion conductive film comprises
an anion conductive polymer having a cation comprising N.

3. The separator of claim 2, wherein the cation of the anion conductive
polymer further comprises a pyridinium group, a pyrrolidinium group, an
ammonium group, an imidazolium group, or a combination thereof.

4. The separator of claim 1, wherein the anion conductive film comprises
an anion conductive polymer, and a cation of the anion conductive polymer
comprises a pyridinium group, a pyrrolidinium group, an ammonium group,
an imidazolium group, or a combination thereof.

6. The separator of claim 1, wherein the anion conductive film has a
thickness in a range from 5 nm to 100 nm.

7. The separator of claim 1, wherein the cation conductive film comprises
a cation conductive polymer having a sulfonic acid group, a carboxylic
acid group, a phosphoric acid group, a phosphonic acid group, or a cation
exchange group of a derivative thereof at the side chain.

8. The separator of claim 1, wherein the cation conductive film has a
thickness in a range from 20 μm to 200 μm.

9. A redox flow battery comprising; an electrode assembly comprising a
separator according to claim 1 and positive and negative electrodes
respectively positioned at both sides of the separator; a positive
electrode supplier configured to supply a positive active material liquid
to a positive electrode; and a negative electrode supplier configured to
supply a negative active material liquid to a negative electrode.

17. The redox flow battery of claim 9, wherein the negative active
material liquid has a concentration of the negative active material in a
range from 1M to 10M.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and the benefit of Korean
Patent Application No. 10-2011-0035830, filed in the Korean Intellectual
Property Office on Apr. 18, 2011, the entire content of which is
incorporated herein by reference.

BACKGROUND

[0002] 1. Field

[0003] This disclosure relates to a separator for a redox flow battery and
a redox flow battery including the same.

[0004] 2. Description of Related Art

[0005] A rechargeable battery is utilized to transform electrical energy
into chemical energy and to store this chemical energy and then to
retransform the chemical energy into electrical energy. Here, a
rechargeable battery having a lighter weight has been actively
researched.

[0006] Recently, a redox flow battery has garnered attention as a
high-capacity and high efficiency rechargeable battery, which may be
appropriate or suitable for a large system such as an electric power
storage system and the like.

[0007] Unlike other batteries, the redox flow battery does not use a solid
as an active material, but rather uses aqueous ions as the active
material and generates energy through an oxidation/reduction reaction of
the aqueous ions at the positive and negative electrodes.

SUMMARY

[0008] An embodiment of the present invention is directed toward a
separator for a redox flow battery that can effectively suppress
cross-over of an active material.

[0009] Another embodiment of the present invention is directed toward a
redox flow battery including the separator.

[0010] According to an embodiment of the present invention, provided is a
separator for a redox flow battery including a cation conductive film,
and an anion conductive film disposed on either side of the cation
conductive film.

[0011] In one embodiment, the anion conductive film includes an anion
conductive polymer having a cation including N. The cation including N
may include a pyridinium group, a pyrrolidinium group, an ammonium group,
an imidazolium group, or a combination thereof.

[0013] The anion conductive film may have a thickness in a range from 5 nm
to 100 nm.

[0014] The cation conductive film may include a cation conductive polymer
including a sulfonic acid group, a carboxylic acid group, a phosphoric
acid group, a phosphonic acid group, or a cation exchange group of a
derivative thereof at the side chain.

[0015] The cation conductive film may have a thickness in a range from 20
μm to 200 μm.

[0016] According to another embodiment of the present invention, provided
is a redox flow battery including an electrode assembly including the
separator and positive and negative electrodes respectively positioned at
both sides of the separator, a positive electrode supplier configured to
supply a positive active material liquid to the positive electrode, and a
negative electrode supplier configured to supply a negative active
material liquid to the negative electrode.

[0017] The positive active material may be a +5-valent to +4-valent
vanadium-based compound, for example, (VO2)2SO4,
VO(SO4), or a combination thereof.

[0018] The positive active material liquid may include a mixed solvent of
sulfuric acid and water.

[0019] The positive active material liquid may have a concentration of the
positive active material in a range from 1M to 10M.

[0020] The negative active material may include a +2-valent to +3-valent
vanadium-based compound, for example, VSO4, V2(SO4)3,
or a combination thereof.

[0021] The negative active material liquid may include a mixed solvent of
sulfuric acid and water.

[0022] The negative active material liquid may have a concentration of the
negative active material in a range from 1M to 10M.

[0023] Hereinafter, further embodiments will be described in more detail.

[0024] According to one embodiment of the present invention, the separator
may effectively suppress cross-over of an active material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 schematically provides the structure of a separator for a
redox flow battery according to an embodiment of the present invention.

[0026]FIG. 2A is a drawing that shows a cross-over phenomenon of an
active material in a conventional separator for a redox flow battery.

[0027]FIG. 2B provides a drawing that shows cross-over suppression of an
active material in a separator for a redox flow battery according to an
embodiment of the present invention.

[0028]FIG. 3 provides a drawing that schematically shows the structure of
a redox flow battery according to another embodiment of the present
invention.

DETAILED DESCRIPTION

[0029] Exemplary embodiments of the present disclosure will hereinafter be
described in more detail. However, these embodiments are only exemplary,
and this disclosure is not limited thereto.

[0030] One embodiment of the present invention provides a separator for a
redox flow battery. In general, a redox flow battery uses positive and
negative active materials in a form of aqueous liquids, and when the
positive and the negative active material liquids are supplied to an
electrode assembly including positive and negative electrodes and a
separator, an oxidation/reduction reaction of ions occurs at the positive
and negative electrodes, thereby generating electrical energy.

[0031] In other words, a tetravalent vanadium ion (e.g., a +4 oxidation
state vanadium ion) is oxidized into a pentavalent vanadium ion (e.g., a
+5 oxidation state vanadium ion), losing an electron and passing a proton
through a separator from the positive electrode to the negative electrode
at the positive electrode during the charge reaction, while a trivalent
vanadium ion (e.g., a +3 oxidation state vanadium ion) accepts the
electron and is converted into a divalent vanadium ion (e.g., a +2
oxidation state vanadium ion), that is, the reduction reaction occurs, at
the negative electrode. On the contrary, an oxidation/reduction reaction,
that is, a redox reaction, may occur during the discharge reaction in
which the oxidation state (oxidation number) of a vanadium ion is
changed.

[0032] In other words, a separator should pass a proton, but block a
cation of positive and negative active materials from moving toward a
counter electrode. However, the separator passes a cation, particularly a
vanadium cation, and is self-discharged, resultantly deteriorating
columbic efficiency.

[0033] According to one embodiment of the present invention, a separator
1, as shown in FIG. 1, may suppress cross-over of a vanadium cation by
forming an anion conductive film 5 on both sides of a cation conductive
film 3. In other words, as shown in FIG. 2A, a conventional separator may
readily pass a cation of an active material, for example a vanadium ion
(a positive electrode: VO2+/VO2.sup.+, a negative electrode:
V2+/V3+), through a cation conductive group (e.g.,
SO3.sup.-) thereof. On the contrary, as shown in FIG. 2B, a
separator according to one embodiment of the present invention has a
cation of an anion conductive film, for example ammonium, and the cation
generates charge rejection, thereby suppressing cross-over of a cation.

[0034] According to one embodiment of the present invention, the anion
conductive film includes an anion conductive polymer having a cation
including N. The cation may include a pyridinium group, a pyrrolidinium
group, an ammonium group, an imidazolium group, or a combination thereof.

[0036] When a porous polymer layer such as polyethylene and the like,
rather than the anion conductive polymer, is formed on both sides of a
cation exchange film, the separator may have somewhat improved mechanical
strength, but may not suppress cross-over.

[0037] In one embodiment, the anion conductive film has a thickness in a
range from 5 nm to 100 nm. In one embodiment, when the anion conductive
film has a thickness within the range, the separator easily passes
protons and thus does not have voltage efficiency deterioration, and
therefore effectively suppresses cation movement of an active material.

[0038] The cation conductive film may include a cation conductive polymer
having a sulfonic acid group, a carboxylic acid group, a phosphoric acid
group, a phosphonic acid group, or a cation exchange group of a
derivative thereof at the side chain. Examples of the cation conductive
polymer may have the cation exchange group at the side chain and include
at least one selected from a fluorine-based polymer, a
benzimidazole-based polymer, a polyimide-based polymer, a
polyetherimide-based polymer, a polyphenylenesulfide-based polymer, a
polysulfone-based polymer, a polyethersulfone-based polymer, a
polyetherketone-based polymer, a polyether-etherketone-based polymer, or
a polyphenylquinoxaline-based polymer.

[0039] In particular, the cation conductive polymer included in the cation
conductive film may include at least one selected from
poly(perfluorosulfonic acid) (commercially available NAFION),
poly(perfluorocarboxylic acid), a copolymer of tetrafluoroethylene and
fluorovinylether including a sulfonic acid group, polyetherketone
sulfide, aryl ketone having the cation exchange group at the side chain,
poly[(2,2'-m-phenylene)-5,5'-bibenzimidazole][poly(2,2'-(m-phenylene)-5,5-
'-bibenzimidazole] having the cation exchange group at the side chain, or
poly(2,5-benzimidazole) having the cation exchange group at the side
chain. In addition, the cation conductive polymer may include a porous
polymer such as porous polyethylene, porous polypropylene, porous
polyvinylchloride, and the like, and the porous polymer has the cation
exchange group at the side chain.

[0040] In one embodiment, the cation conductive film has a thickness in a
range from 20 μm to 200 μm. In one embodiment, when a cation
conductive film has a thickness within the range, the separator has high
energy efficiency. In another embodiment, when a cation conductive film
has a thickness of less than 20 μm, the separator has decreased
resistance against protons transference and thus has increased voltage
efficiency. However, the separator may have more pin-holes on the
conductive film due to the pressure generated when an active material
liquid is injected into positive and negative electrodes, and thus has
higher cross-over, thereby deteriorating columbic efficiency. In addition
and in one embodiment, when a cation conductive film has a thickness of
more than 200 μm, the cation conductive film has a higher mechanical
strength and less possibility of generating a pin-hole, and also lower
cross-over of an active material and thus higher columbic efficiency.
However, the cation conductive film may have increased resistance against
protons transference, thereby decreasing voltage efficiency.

[0041] According to one embodiment of the present invention, a separator
is fabricated by adding an anion conductive polymer to a solvent to
prepare an anion conductive polymer liquid and coating the anion
conductive polymer liquid on both sides of a cation conductive film. The
solvent may include methanol, ethanol, isopropyl alcohol, acetone,
N-methylpyrrolidone, n-propylalcohol, tetrahydrofuran, water, or a
combination thereof. The coating may include spray coating, dip coating,
doctor blade coating, comma bar coating, slot die coating, and the like.
In one embodiment, the anion conductive polymer in the anion conductive
polymer liquid has a concentration in a range from 3 wt % to 20 wt %. In
one embodiment, when the anion conductive polymer in the anion conductive
polymer liquid has a concentration within the range, the coating is
uniformly formed, thereby forming an anion conductive polymer layer with
an appropriate thickness without increasing resistance.

[0042] According to another embodiment of the present invention, provided
is a redox flow battery including an electrode assembly including the
separator and positive and negative electrodes respectively at (on) both
sides of the separator, a positive electrode supplier configured to
supply a positive active material liquid to the positive electrode, and a
negative electrode supplier configured to supply a negative active
material liquid to the negative electrode.

[0043] The positive active material may be a +5-valent (pentavalent) to
+4-valent (tetravalent) vanadium-based compound (e.g., a +5 oxidation
state to +4 oxidation state vanadium-based compound), for example,
(VO2)2SO4, VO(SO4), or a combination thereof.

[0044] The positive active material liquid may include a mixture of
sulfuric acid and water, that is, a sulfuric acid aqueous solution, as a
solvent. In one embodiment, the mixture of sulfuric acid and water, that
is, a sulfuric acid aqueous solution, includes a sulfuric acid with a
concentration in a range from 1M to 5M. In one embodiment, when the
sulfuric acid aqueous solution has a concentration within the range, it
has appropriate solubility against the active material, and thus has
appropriate ion conductivity and increases viscosity of the active
material liquid and resultantly may not bring about an overvoltage
problem.

[0045] In one embodiment, the positive active material liquid has a
concentration in a range from 1M to 10M (of the positive active material
in the liquid). In one embodiment, when the positive active material
liquid has a concentration within the range, it can bring about high
energy density and high power density. In one embodiment, when the
positive active material liquid has a concentration of less than 1M, the
active material included in the liquid is too little of an amount per
unit volume, thereby decreasing energy density. On the contrary and in
another embodiment, when the positive active material liquid has a
concentration of more than 10M, the active material liquid has sharply
increased viscosity and thus a remarkably decreased oxidation/reduction
reaction speed, thereby decreasing power density.

[0046] The negative active material may be a +2-valent (divalent) to
+3-valent (tetravalent) vanadium-based compound (e.g., a +2 oxidation
state to +3 oxidation state vanadium-based compound), for example,
VSO4, V2(SO4)3, or a combination thereof.

[0047] The negative active material liquid may include a mixture of
sulfuric acid and water, that is, a sulfuric acid aqueous solution, as a
solvent like the positive active material liquid.

[0048] In one embodiment, the negative active material liquid has a
concentration in a range from 1M to 10M. In one embodiment, when the
negative active material liquid has a concentration within the range, it
brings about high energy density and high power density. In one
embodiment, when the negative active material liquid has a concentration
of less than 1M, the active material in the liquid is of too less of an
amount per unit volume, thereby decreasing energy density. On the other
hand and in another embodiment, when the negative active material liquid
has a concentration of more than 10M, the active material liquid has
sharply increased viscosity and thus a sharply decreased
oxidation/reduction reaction speed, thereby decreasing power density.

[0049] The positive and negative electrodes may be made of a conductive
substrate. In addition, the electrodes may further include a polar plate
on one side of the conductive substrate, that is, on the side facing
oppositely away from a separator.

[0050] The conductive substrate may include carbon paper, carbon cloth,
carbon felt, or metal cloth (a porous film made of fiber-type metal or a
metal film formed on the surface of a polymer fiber cloth), but is not
limited thereto. Furthermore, the conductive substrate may be porous.

[0051] In addition, the polar plate may be made of graphite. The polar
plate may have a channel.

[0052]FIG. 3 schematically shows the structure of the redox flow battery.
As shown in FIG. 3, a redox flow battery 20 includes an electrode
assembly including a separator 22 and a positive electrode 24 and a
negative electrode 26 respectively at (on) both sides of the separator
22, a positive electrode tank 28 configured to supply a positive active
material to the positive electrode 24, and a negative electrode tank 30
configured to supply a negative active material to the negative electrode
25. The positive active material stored in the positive electrode tank 28
is delivered to the positive electrode 24 through a pump 51 and a
positive active material inlet 31, and is then sent back to the positive
electrode tank 28 through a positive active material outlet 41 when a
redox reaction is completed. The negative active material stored in the
negative electrode tank 30 is delivered to the negative electrode 26
through a pump 52 and a negative active material inlet 32, and is then
sent back to the positive electrode tank 30 through a positive active
material outlet 42 when a redox reaction is completed.

[0053] The following examples illustrate this disclosure in more detail.
These examples, however, should not be interpreted as limiting the scope
of this disclosure.

Example 1

[0054] A poly(styrene-b-2-vinylpyridinium chloride) anion conductive
polymer is added to an n-propanol/tetrahydrofuran mixed solvent in a
volume ratio of 1/1 to prepare an anion conductive liquid with a
concentration of 5 wt % of the anion conductive polymer in the anion
conductive liquid.

[0055] The anion conductive polymer liquid is spray-coated to be 5 nm
thick on both sides of a 180 μm-thick NAFION 117 cation conductive
film, thereby fabricating a separator having a 5 nm-thick anion
conductive film at (on) both sides of the cation conductive film.

[0056] Next, a carbon felt positive electrode and a negative electrode are
respectively positioned at (on) both sides of the separator, and graphite
polar plates are respectively positioned at (on) one side of the positive
electrode facing oppositely away from the separator and at (on) one side
of the negative electrode facing oppositely away from the separator. The
separator is clamped with the positive and negative electrodes and the
graphite polar plates, thereby fabricating an electrode assembly. Herein,
the electrode area is 6 cm2.

[0058] The electrode assembly and the positive and negative active
material solutions are used to fabricate a redox flow battery cell with a
structure provided in FIG. 3.

Example 2

[0059] A redox flow battery cell is fabricated according to the same
method as Example 1 except for using a separator fabricated by coating
the anion conductive polymer liquid according to Example 1 to be 10 nm
thick on both sides of a 180 μm-thick NAFION 117 cation conductive
film to form a 10 nm-thick anion conductive film on both sides thereof.

Example 3

[0060] A redox flow battery cell is fabricated according to the same
method as Example 1 except for using a separator fabricated by coating
the anion conductive polymer liquid according to Example 1 to be 100 nm
thick on both sides of a 180 μm-thick NAFION 117 cation conductive
film to form a 100 nm-thick anion conductive film on both sides thereof.

Comparative Example 1

[0061] A redox flow battery cell is fabricated according to the same
method as Example 1 except for using a 180 μm-thick NAFION 117 cation
conductive film as a separator.

Comparative Example 2

[0062] A redox flow battery cell is fabricated according to the same
method as Example 1 except for using a separator fabricated by coating
the anion conductive polymer liquid to be 500 nm thick on both sides of a
180 μm-thick NAFION 117 cation conductive film to form a 500 nm-thick
anion conductive film on both sides thereof.

Comparative Example 3

[0063] A redox flow battery cell is fabricated according to the same
method as Example 1 except for using a separator fabricated by coating
the anion conductive polymer liquid to be 1000 nm thick on both sides of
a 180 μm-thick NAFION 117 cation conductive film to form a 1000
nm-thick anion conductive film on both sides thereof.

[0064] The redox flow battery cells according to Examples 1 to 3 and
Comparative Examples 1 to 3 are measured regarding voltage efficiency,
columbic efficiency, and energy efficiency by injecting the positive and
negative active material solutions respectively in an amount of 30 ml
into the electrode assembly and charging and discharging the battery
cells with a current density of 50 mA/cm2.

[0065] Herein, the voltage efficiency is calculated according to the
following Equation 1.

Voltage efficiency(%)=(an average voltage during the discharge/an
average voltage during the charge)×100

[0067] As shown in Table 1, the redox flow battery cells, each including a
separator having a 5 nm to 100 nm-thick anion conductive film according
to each of Examples 1 to 3, have better columbic efficiency and energy
efficiency than the one including no anion conductive film according to
Comparative Example 1. In addition, the redox flow battery cells, each
including a separator having more than 100 nm anion conductive film
according to each of Comparative Examples 2 and 3, have improved columbic
efficiency, but deteriorated voltage efficiency and energy efficiency,
and also have increased film resistance compared with the ones according
to Examples 1 to 3.

[0068] While this disclosure has been described in connection with what is
presently considered to be practical exemplary embodiments, it is to be
understood that the invention is not limited to the disclosed
embodiments, but, on the contrary, is intended to cover various
modifications and equivalent arrangements included within the spirit and
scope of the appended claims. Therefore, the aforementioned exemplary
embodiments should be understood to be exemplary in every way, but not
limited thereto.